Fix alter column issue in Postgres

[hishamco]

Fixes #483

[hishamco]

While I'm working on this I found two issues:

1- modify is incorrect as discussed on the related issue
2- Also, we miss Type when we change the column datatype, seems no one alter the column previously in Postgres :)

One more thing that confused me, I'm expecting to not specify the type during the alteration, if we don't the default bytea is added to the column which is wrong

[hishamco]

Nah, SQLite test is failed!! I presume because it doesn't support alteration if I'm not wrong, @sebastienros shall we move the test to PostgresTests?

[MikeKry]

@hishamco

I would also expect that it is failing because sqlite does not support alter column..

Can this be merged? It is vital to have possibility to change column size in postgres.

[hishamco]

@sebastienros could you please reply on my comment so we can finalize this PR?

[sebastienros]

Add [Skip] in the Sqlite tests then. There are some tests already that are ignored by db.

[hishamco]

Ready to merge Seb